1. Field of the Invention
The present invention relates to an electrophotographic recording material composed of an electrically conductive substrate onto which three selenium-containing layers are consequently applied.
2. Background of the Art
Electrophotographic recording materials include photoconductive materials and are employed in photocopying processes which are essentially based on the fact that the photoconductive materials employed change their electrical resistance when irradiated with actinic light, i.e., activating radiation. For many applications, the electrophotographic recording material is composed simply of a single photoconductive layer applied to an electrically conductive substrate. In the unexposed state, the photoconductor has a relatively high dark resistance and can therefore be electrically charged. Upon exposure to actinic light, the charge is dissipated in the exposed locations and, ideally, the residual potential is zero. When exposure to actinic light is a pattern-wise exposure achieved, for example, by projecting an optical image onto the charged photoconductor, a latent, electrically charged image remains which corresponds to the dark areas of the optical image. This latent image is then developed by any of several known methods, such as by means of contact with a particulate toner material, and the developed image is transferred to paper.
An electrophotographic recording material and the photoconductor employed therein must meet the requirements that the dark resistance of the photoconductor be very high, while after exposure, the residual potential be as low as possible. It is also frequently desirable for the electrophotographic recording material to exhibit panchromatic behavior, i.e., to be sensitive to activating radiation spanning at least the entire visible spectral range. For some applications, it is preferable that the panchromatic behavior be extended to include the infrared (IR) spectral range, out to, for example, 900 nm. Recording material having such an IR-extended panchromatic response can be activated by the radiation from an IR solid state laser as employed, for example, in data output devices. In addition to the above-mentioned characteristics, the recording material must also have, inter alia, good thermal stability and exhibit little charge/discharge fatigue on recycling.
To satisfy as many of the above requirements as possible and to simultaneously provide the generally directly proportional properties of high dark resistance and low residual potential, single-layered photoconductive materials have been replaced by multi-layered photoconductive materials. Recording materials employing three photoconductive layers, on which the present invention is based, are disclosed in, for example, German Laid-open Patent Application No. 2,615,624 and German Patent No. 1,597,882, the disclosures of which are herein incorporated by reference. The first layer adjacent the substrate functions as a charge carrier transporting layer since, upon exposure, it is constituted so as to facilitate the movement of exposure-generated charge carriers from the center layer to the substrate. The center, or second layer, is constituted so as to function as a charge carrier generating layer since charge carriers are produced in it upon exposure to activating radiation. It is these charge carriers which are responsible for transporting electrostatic charge from the exposed location through the photoconductor to the substrate, which substrate is maintained at a lower potential, such as ground potential. Finally, the third layer generally is constituted so as to function primarily as a protective layer which is provided, inner alia, to improve the mechanical characteristics of the recording material, such as to improve wear resistance.
In German Patent No. 1,597,882, the first layer is composed of an amorphous selenium-arsenic alloy; the second layer, an amorphous selenium-tellurium alloy; and the third layer, an amorphous selenium-arsenic alloy. This triple-layer arrangement provides a panchromatically-sensitive recording material, which is thermally- and humidity-stable and which exhibits only slight evidence of charge/discharge fatigue on recycling. This prior art recording material, however, has a relatively high residual potential and has a fatigue characteristic upon cycling, i.e., a reduced charge acceptance and/or a reduced contrast potential upon charge/discharge recycling, which is still excessive for many applications.